The present invention relates to plastic post and railing systems which use components of extruded rigid plastics materials and of the general type disclosed in U.S. Pat. Nos. 4,540,160, 4,722,514, 4,809,955, 5,161,783 and 5,651,534. In any such system, it is desirable for the vertical posts and the horizontal rails to provide high strength and durability in addition to a clean and neat appearance with no maintenance. It is also desirable for the system to be designed for rapid and convenient installation with a minimum number of components and fasteners while also using economically produced components. While some of these features are provided by some of the systems disclosed in the above patents, none of the systems provide all of the desirable features.